Liquid crystal display device

ABSTRACT

A back light unit includes a light guide plate having at least one light introducing portion and a light exit portion, the front surface of the light exit portion being a light exit surface, and a light source facing the light guide plate and emitting light in a light emitting direction. The at least one light introducing portion includes a first portion and a second portion which is connected to the light exit portion, and in which the second portion spreads wider than the first portion in plan view.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation application of U.S.application Ser. No. 13/116,048 filed on May 26, 2011 which claimspriority from Japanese application JP 2010-121967 filed on May 27, 2010,the content of which is hereby incorporated by reference into thisapplication.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a liquid crystal display device.

2. Description of the Related Art

In recent years, liquid crystal display devices are widely used asinformation display devices for portable devices. In order to reduce thesize of the portable devices, the liquid crystal display devices need tobe reduced in entire thickness.

However, the liquid crystal display device is not self-luminous, andhence a planar lighting unit called “backlight” is often required. It isknown that the liquid crystal display device used for portable devicesemploys a system, as a planar lighting unit, in which a light source isprovided on a side surface of a light guide plate for the purpose ofreducing the thickness of the backlight. In the planar lighting unit asdescribed above, the light guide plate is provided with an appropriatestructure for uniformly scattering light introduced from the sidesurface onto the front surface thereof. Further, in the planar lightingunit as described above, there must be given a consideration to theavoidance of undesired light from being reflected on a screen.

For example, JP 07-43710 A discloses a liquid crystal display device, inwhich a light guide plate is provided with a plurality of grooves on asurface thereof, the plurality of grooves each having projectionsprojecting outward on both sides of the aperture of the groove.According to the invention disclosed in JP 07-43710 A, light which hasentered the projections goes outside to be diffusely reflected by areflective member, and then enters the light guide plate again.

Meanwhile, JP 2005-251687 A discloses a liquid crystal display device,in which light sources are provided on a side surface of a light guideplate, and a light blocking member is provided on a flexible printedcircuit board so as to prevent a color of the flexible printed circuitboard from being mixed with colors on a screen.

In general, a light guide plate is made of a transparent thermoplasticresin, such as polycarbonate or polymethylmethacrylate, and is formedthrough projection molding. However, when the projection molding methodis employed, it has been difficult to obtain a light guide plate with athickness smaller than a certain degree, for example, the thickness of 1mm or less, due to the reason that the resin is insufficiently filledinto the mold or it is difficult to remove the finished product from themold. For that reason, the applicant of the present invention isconsidering adding a surface structure on a surface of a sheet made of athermoplastic material by transfer with the use of a mold. At this time,an appropriate structure which scatters light is formed on the surfaceof the sheet made of the thermoplastic material described above. At thesame time, a light introducing structure is formed thereon, which guideslight introduced from the side surface of the light guide plate insidethe light guide plate. After that, a periphery portion of the sheet madeof the thermoplastic material is cut out by a method such as punching,to thereby obtain a light guide plate with a desired dimension.

However, in the method described above, the dimension accuracy in thecutout process by a method such as punching is not always high, and thegeometric position at the end surface of the light introducing structuremay be easily misaligned. Therefore, when the liquid crystal displaydevice is assembled, light from a light source such as a light-emittingdiode (LED) cannot be sufficiently introduced into the light introducingstructure, and there is a fear that the use efficiency of the light fromthe light source is decreased.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above-mentionedcircumstances, and it is an object of the present invention to provide aliquid crystal display device including a light guide plate which isobtained by cutting out a periphery portion of a sheet made of athermoplastic material by a method such as punching and which cansufficiently introduce light from a light source into a lightintroducing structure.

Among aspects of the invention disclosed in the present application, therepresentative ones are briefly described as follows.

(1) A back light unit includes: a light guide plate having at least onelight introducing portion and a light exit portion, the front surface ofthe light exit portion being a light exit surface, and a light sourcefacing the light guide plate and emitting light in a light emittingdirection. The at least one light introducing portion includes a firstportion and a second portion which is connected to the light exitportion, and in which the second portion spreads wider than the firstportion in plan view.

(2) In the back light unit as described in Item (1) of the presentinvention, a lens is provided in a light incident surface of the lightintroducing portion, the lens being configured to refract and scatterincident light.

(3) The back light unit as described in Item (2) of the presentinvention including a light entering portion having a plurality of lightintroducing portions and light non-introducing portions at the same sideof the light guide plate, the light non-introducing portions beingthinner than the light introducing portions.

(4) In the back light unit as described in Item (1) of the presentinvention, the front width of the first portion is smaller than the backwidth thereof.

(5) In the back light unit as described in Item (1) of the presentinvention, the first portion has a thickness which is thicker than athickness of the light exit portion, and the thickness of the secondportion is distributed in between the thickness of the first portion andthe light exit portion.

(6) In the back light unit as described in Item (2) of the presentinvention, the front surface of the second portion is an inclinedsurface, and an inclination of the inclined surface is greater than aninclination of the front surface of the first portion.

(7) A liquid crystal display including an optical switching member whichincludes a first substrate and a second substrate sandwiching a liquidcrystal layer, and a back light unit according to any of the Item (1) to(6) which guides light to the optical switching member.

According to the invention disclosed in the present application asdescribed above, it is possible to obtain the liquid crystal displaydevice including the light guide plate which is obtained by cutting outthe peripheral portion of the sheet made of the thermoplastic materialby a method such as punching and which can sufficiently introduce thelight from the light source into the light introducing structure.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a plan view illustrating a liquid crystal display deviceaccording to an embodiment of the present invention;

FIG. 2A is a schematic cross-sectional view illustrating alight-emitting diode as a light source;

FIG. 2B is a front view of the light-emitting diode as the light sourceviewed from the light exit surface side;

FIG. 3A is a schematic plan view of a light guide plate;

FIG. 3B is a schematic side view of the light guide plate;

FIG. 4 is a view illustrating light reflected by a groove;

FIG. 5 is an enlarged perspective view of a light entering portion ofthe light guide plate;

FIG. 6A is a view illustrating a lens having a cross section in atriangular shape;

FIG. 6B is a view illustrating a lens having a cross section in a shapein which a plurality of semicircular cylindrical lenses are connected toone another;

FIG. 7 is an enlarged front view including a first portion viewed from adirection indicated by the arrow VII of FIG. 5;

FIG. 8 is a schematic plan view of a sheet made of a thermoplasticmaterial in a middle of manufacturing the light guide plate;

FIG. 9A is a cross-sectional view taken along the line IXA-IXA of FIG.8, which illustrates a state during a cutout process;

FIG. 9B is a cross-sectional view taken along the line IXB-IXB of FIG.8, which illustrates the state during the cutout process;

FIG. 10 is a view illustrating a modified example of the light enteringportion;

FIG. 11 is a view illustrating an installation structure for installingthe light-emitting diode in the light guide plate; and

FIG. 12 is a cross-sectional view of the liquid crystal display device.

DETAILED DESCRIPTION OF THE INVENTION

In the following, a preferable embodiment of the present invention isdescribed with reference to the accompanying drawings.

FIG. 1 is a plan view illustrating a liquid crystal display device 100according to this embodiment. The liquid crystal display device 100includes an optical switching member 1, a backlight 110, and a controlcircuit 80. Signals required for display on the liquid crystal displaydevice 100 and a power supply voltage are supplied from the controlcircuit 80. The control circuit 80 is mounted on flexible printedcircuits 70, and signals are delivered to the optical switching member 1via wiring 71 and a terminal 75.

The backlight 110 includes a light guide plate 120, a light-emittingdiode 150 which is a light source, and a housing 190. The backlight 110is provided to irradiate the optical switching member 1 with light. Inthe optical switching member 1, displays are performed by controlling anamount of transmitted light which is radiated from the backlight 110.The backlight 110 is provided in an overlaid state on the opticalswitching member 1 when viewed from a viewer, but in FIG. 1, thebacklight 110 is juxtaposed with the optical switching member 1 for easein understanding. Note that, hereinafter in this specification, adirection in which the liquid crystal display device 100 faces a vieweris referred to as front side while a direction opposite thereto isreferred to as back side, and a surface of the liquid crystal displaydevice 100 on the front side is referred to as front surface whileanother surface thereof on the back side is referred to as back surface.The backlight 110 is generally disposed on the back side of the opticalswitching member 1, whereas the backlight 110 may be disposed on thefront side alternatively. When the backlight 110 is disposed on thefront side, the optical switching member 1 controls the amount ofreflected light which is radiated from the backlight 110.

The light guide plate 120 has a substantially rectangular form, and thelight-emitting diode 150 is provided as being opposed to a lightincident surface 125, which is a side surface of the light guide plate120. Reference numeral 160 denotes flexible printed circuits forelectrical connection among a plurality of light-emitting diodes 150.The flexible printed circuits 160 and the control circuit 80 areelectrically connected to each other by wiring 161.

Light that has entered the light guide plate 120 from the light incidentsurface 125 exits from the light exit surface 121, which is the frontsurface of the light guide plate 120. In a section between the lightincident surface 125 and the light exit surface 121, a light enteringportion 124 is provided, which has a role of efficiently transmittingthe light from the light-emitting diode 150 to the light exit surface121. The light entering portion 124 includes at least one (three inFIG. 1) light introducing portion 134 and a light non-introducingportion 135, which is a region sandwiched by the light introducingportions 134 or a region sandwiched by the light introducing portion 134and an edge portion of the light guide plate 120. The light enteringportion 124 is described in detail later.

Next, the optical switching member 1 is described. The optical switchingmember 1 includes two substrates, namely, a TFT substrate 2 and a colorfilter substrate 3, and a liquid crystal composition is sandwichedbetween the overlaid two substrates. The TFT substrate 2 has a pluralityof pixel portions 8, and a pixel electrode 12 is provided in each pixelportion 8. The plurality of pixel portions 8 are aligned in a gridpattern in a display area 9. The pixel portions 8 function as opticalswitching elements for controlling an amount of transmitted light fromthe backlight 110 so as to function as pixels in the liquid crystaldisplay device 100, to thereby form an image in the display area 9. Notethat, in FIG. 1, only one pixel portion 8 is illustrated in order toevade complexity of the drawing.

FIG. 1 illustrates gate signal lines (also referred to as scanning line)21 extending in an X direction of FIG. 1 and provided in parallel toeach other in a Y direction of FIG. 1 and drain signal lines (alsoreferred to as video signal line) 22 extending the Y direction andprovided in parallel to each other in the X direction. The gate signallines 21 and the drain signal lines 22 cross over with each other. Thepixel portion 8 is formed in a region surrounded by the gate signallines 21 and the drain signal lines 22.

A switching element 10 such as a thin film transistor (TFT) is providedin the pixel portion 8. Control signals are supplied from the gatesignal line 21 to turn ON/OFF the switching element 10. When theswitching element 10 is turned ON, a video signal transmitted via thedrain signal line 22 is supplied to the pixel electrode 12.

The drain signal line 22 is connected to a drive circuit 5, and a videosignal is output from the drive circuit 5. The gate signal line 21 isconnected to a drive circuit 6, and a control signal is output from thedrive circuit 6. The gate signal line 21, the drain signal line 22, andthe drive circuits 5 and 6 are formed on the same TFT substrate 2. Aconfiguration is also allowable in which the drive circuits 5 and 6 andthe control circuit 80 are formed on the same semiconductor chip.

Note that, the liquid crystal driving system in the optical switchingmember 1 is not specifically limited. Any publicly-known system, such asa twisted nematic (TN) system, a vertical alignment (VA) system, or anin-plane switching (IPS) system, may be employed without any problem.

FIGS. 2A and 2B are schematic views illustrating the light-emittingdiode 150 as a light source. FIG. 2A is a schematic cross-sectional viewillustrating the same, and FIG. 2B is a front view illustrating thelight-emitting diode 150 viewed from the light exit surface side.

The light-emitting diode 150 has a structure in which a light-emittingdiode chip 151 as a light-emitting portion is mounted on a chipsubstrate 154. The light-emitting diode chip 151 has a p-n junction, andwhen a voltage is applied to the p-n junction, light having a particularwavelength is emitted. A p electrode (anode) 158 is provided in a p-typesemiconductor layer, and an n electrode (cathode) 159 is provided in ann-type semiconductor layer, those layers constituting the p-n junction.

A wire 152 is connected to each of the p electrode 158 and the nelectrode 159. The wire 152 electrically connects a chip terminal 153provided to connect the light-emitting diode 150 to the outside, to eachof the p electrode 158 and the n electrode 159.

In some cases, a fluorescence emitting portion 156 may be provided onthe light exit surface side of the light-emitting diode chip 151. Thefluorescence emitting portion 156 has a function of converting awavelength of light emitted from the light-emitting diode chip 151. Areflection portion 155 reflects light forward. A light exit surface 157from which light exits is formed on a front side of the light-emittingdiode 150.

FIGS. 3A and 3B are schematic views illustrating the light guide plate120. FIG. 3A is a schematic plan view of the light guide plate 120 whileFIG. 3B is a schematic side view of the same. In both of the figures, inorder to show the positional relations, the light-emitting diode 150 andthe flexible printed circuits 160 are also illustrated. The light guideplate 120 has a substantially rectangular form as illustrated in FIG.3A, and has the light exit surface 121 as a front surface and a backsurface 122 as illustrated in FIG. 3B. The light guide plate 120 is madeof an optically transmissive thermoplastic material, such aspolycarbonate or polymethylmethacrylate, and has a sheet-like form. Thethickness of the light guide plate 120 may preferably be in the rangefrom 1.0 mm to 0.1 mm. In this case, the thickness of the light guideplate 120 corresponds to a distance between the light exit surface 121and the back surface 122.

In FIG. 3B, the cross section of the light guide plate 120 issubstantially rectangular, and the thickness of the light introducingportion 134 included in the light entering portion 124 provided in thesection between the light incident surface 125 and the light exitsurface 121 is larger than the thickness of the light guide plate 120.Further, the inclined surface 127 in the top of the light introducingportion 134 connects the light incident surface 125 and the light exitsurface 121 smoothly and continuously. Further, as illustrated in FIG.3A, the inclined surface 127 has a fan-shaped form which widens in adirection away from the light-emitting diode 150 in the optical axisdirection thereof (X direction of FIG. 3A) in plan view of the lightguide plate 120. The structure of the light introducing portion 134 isdescribed in detail later, but with this structure, light from thelight-emitting diode 150 thicker than the light guide plate 120 may beeffectively guided inside the light guide plate 120. Therefore, thethickness of the light incident surface 125 in the light introducingportion 134 is preferred to be substantially equal to the thickness ofthe light-emitting surface of the light-emitting diode 150. Meanwhile,the thickness of the light non-introducing portion 135 is smaller thanthe thickness of the light guide plate 120. This is mainly based on amanufacturing reason as described later. The structures of the lightintroducing portion 134 and the light non-introducing portion 135 aredescribed in detail later.

FIGS. 3A and 3B illustrate positional relations among the light guideplate 120, the light-emitting diode 150, and the flexible printedcircuits 160. The light incident surface 125 is provided on at least oneside of the light guide plate 120, and a plurality of the light-emittingdiodes 150 are provided at positions adjacent to the light incidentsurface 125. The light-emitting diodes 150 are arranged on the lowerside of the flexible printed circuits 160 along the light incidentsurface 125.

An intermediate member such as a double sided adhesive tape (not shown)is provided on the light guide plate 120 side of the flexible printedcircuits 160, and the light-emitting diode 150 is positioned withrespect to the light incident surface 125 by adhering and fixing theflexible printed circuits 160 to the light guide plate 120.

Next, description is given of a light beam 140 radiated from thelight-emitting diode 150 with reference to FIG. 3B. The light beam 140radiated from the light-emitting diode 150 enters the light guide plate120 from the light incident surface 125. The light guide plate 120 has alarger index of refraction than that of air, and hence the light beam140 that has reached the light incident surface 125 at an angle largerthan a specific angle with respect to the perpendicular direction of thelight incident surface 125 is reflected, whereas the light beam 140 thathas reached the light incident surface 125 at an angle smaller than thespecific angle comes into the inside of the light guide plate 120.

The light exit surface 121 and the back surface 122 of the light guideplate 120 are substantially orthogonal to the light incident surface125, respectively, and the light beam 140 coming into the inside of thelight guide plate 120 propagates forward in the light guide plate 120while being totally reflected on the light exit surface 121 and the backsurface 122 of the light guide plate 120 repetitively. The back surface122 is provided with a V-shaped groove 126 as a reflection portion. Apart of the light beam 140 propagating in the light guide plate 120 isreflected toward the light exit surface 121 by the grooves 126 providedon the back surface 122, and exits through the light exit surface 121.The groove 126 is provided in a direction substantially orthogonal tothe optical axis direction of the light-emitting diode 150.

Next, description is given of the light beam 140 reflected by the groove126 with reference to FIG. 4. FIG. 4 illustrates, in addition to thelight guide plate 120, prism sheets 112 and 113, a diffusion plate 114,and a reflective sheet 115. Further, the light-emitting diode 150 isassumed to be provided on the left of FIG. 4. The groove 126 is asurface structure formed on the back surface 122 of the light guideplate 120, and includes a first light reflection plane 129 which facesthe light-emitting diode 150 and a second light reflection plane 130which is on a side opposite to the first light reflection plane 129. Thefirst light reflection plane 129 and the second light reflection plane130 are each inclined by 1 to 35 degrees with respect to the backsurface 122. In this embodiment, the light beam 140 that has radiatedfrom the light-emitting diode 150 and propagated in the light guideplate 120 is reflected mainly by the first light reflection plane 129and an angle of the traveling direction of the light beam 140 is changedso that the light beam 140 may exit through the light exit surface 121.In other words, as described above, the light beam 140 propagatesforward along the optical axis direction of the light-emitting diode 150as being totally reflected repetitively in the light guide plate 120,and then the traveling direction of the light beam 140 is changed mainlyby the first light reflection plane 129 to an angle at which the lightbeam 140 is allowed to exit, with the result that the light beam 140exits through the light exit surface 121 of the light guide plate 120.

Note that, when the light-emitting diode 150 as a light source isadditionally provided also on the right of FIG. 4, it is obvious thatthe second light reflection plane 130 functions similarly to the firstlight reflection plane 129.

Further, in this embodiment, the light exit surface 121 of the lightguide plate 120 also has a groove 132 provided thereon. The groove 132is a surface structure similar to the groove 126 of the back surface122, and is formed substantially orthogonally to the groove 126. Thegroove 132 has a function of refracting the light beam 140 reflected bythe first light reflection plane 129 toward the front side of the lightguide plate 120. The light beam 140 radiated from the light guide plate120 is diffused by the diffusion plate 114, and is changed its directionby the prism sheets 113 and 112 toward the front side of the light guideplate 120. The prism sheets 113 and 112 each are a transparent sheethaving a triangular prism-shaped surface structure formed thereon, andare disposed such that the triangular prism-shaped surface structuresare orthogonally oriented. The reflective sheet 115 reflects the lightbeam 140 that has been radiated from the light guide plate 120, towardthe back surface of the light guide plate 120, to thereby introduce thelight beam 140 into the light guide plate 120 again. Note that, theprism sheet 113 and the groove 132 are similar to each other inoperational effect, and hence one of the prism sheet 113 and the groove132 may be omitted when unnecessary.

Further, in this embodiment, the light guide plate 120 is thin andeasily deformed. In view of this, projections 133 projected from theback surface 122 are formed on both sides of the groove 126, to therebyprevent intimate adhesion of the light guide plate 120 to the reflectivesheet 115. With this configuration, there is produced an effect ofsuppressing unevenness in luminance distribution or light leakage, whichmay be caused otherwise when the light guide plate 120 is intimatelyadhered to the reflective sheet 115.

Next, the structure of the light entering portion 124 of the light guideplate 120 is described in detail. FIG. 5 is an enlarged perspective viewof the light entering portion 124 of the light guide plate 120.

In the light entering portion 124, the light introducing portion 134,which corresponds to a protruded portion when viewed from the lightincident surface 125 side, and the light non-introducing portion 135,which corresponds to a recessed portion when viewed therefrom, arealternately formed. The light incident surface 125 has a concave-convexshape when viewed from a normal direction thereof. Note that, in thisembodiment, the concave-convex shape described above is formed on thefront side of the light guide plate 120, but instead, the concave-convexshape may be formed on the back side of the light guide plate 120.Further, as described above, the light introducing portion 134 has athickness larger than the thickness of the light guide plate 120, andthe light non-introducing portion 135 has a thickness smaller than thethickness of the light guide plate 120.

Further, a lens 123 is provided in a portion of the light incidentsurface 125 corresponding to the light introducing portion 134. The lens123 functions to refract light entering from the light introducingportion 134 so as to scatter the light, and to uniformly provide theincident light to the entire surface of the light guide plate 120. Thelens 123 may be in various shapes, but is preferred to be in a shapedrawn into the thickness direction of the light guide plate 120. Thereason is that, as described later, when the light guide plate 120 isformed by being punched out from the sheet in the thickness direction,the lens 123 may be simultaneously formed. Therefore, the lens 123 ispreferred to be formed in a shape which is easy to punch out, andexamples of the lens 123 include a lens which is triangle in crosssection as illustrated in FIG. 6A, and a lens formed of a plurality ofsemicircular cylindrical lenses connected to one another as illustratedin FIG. 6B. In view of the ease of fabrication, there may be preferablyselected a lens with round corners, whereas the lens 123 may be in ashape of a general lenticular lens or in a shape with a sawtoothpattern.

Returning to FIG. 5, the light introducing portion 134 includes a firstportion 136 and a second portion 137 from the light incident surface 125in a direction away from the light source (that is, the optical axisdirection). The first portion 136 has a form extending from the lightincident surface 125 in the direction away from the light source whilekeeping a substantially constant width. Further, the second portion 137extending from the first portion 136 has a form which widens in thedirection away from the light source. Still further, the thickness ofthe first portion 136 is substantially constant along the direction awayfrom the light source, whereas the thickness of the second portion 137changes so as to smoothly continue from the first portion 136 to thelight exit surface 121 which is the front surface of the light guideplate 120. In this embodiment, the front surface of the second portion137 is the inclined surface 127 connecting the first portion 136 and thelight exit surface 121. Note that, as in this embodiment, the entirefront surface of the second portion 137 may be the inclined surface 127,or only a part thereof may be the inclined surface 127.

FIG. 7 is an enlarged front view including the first portion 136 viewedfrom a direction indicated by the arrow VII of FIG. 5. As illustrated inFIG. 7, the protruded portion in the light incident surface 125corresponds to the first portion 136 of the light introducing portion134. In this case, both side surfaces 138 of the first portion 136 areslightly inclined with respect to a direction toward the front surfaceof the light guide plate 120 (upward direction in FIG. 7). The directionof this inclination is a direction in which the width of the firstportion 136 decreases toward the front surface direction of the lightguide plate 120. Further, the angle θ of this inclination is preferablyabout 10 degrees or smaller, more preferably 5 degrees or smaller, andstill more preferably between 1 degree to 2 degrees. This reason isthat, if the angle θ is too large, the entered light beam is reflectedtoward the back side of the light guide plate 120, and hence it becomesdifficult to obtain uniform lighting in the entire light guide plate120. Meanwhile, a slight inclination is necessary to obtain a draftangle from the mold used when the first portion 136 is formed by thermaltransfer as described later, and obtain a self aligning function inpositioning when the light guide plate 120 is punched out from thesheet.

Next, a manufacturing method for the light guide plate 120 is described.The light guide plate 120 is formed of a sheet 170 made of athermoplastic material having a thickness which is substantially equalto the thickness of the light guide plate 120. Steps thereof aresequentially described below with reference to FIG. 8.

In a first step, the sheet 170 made of the thermoplastic material isheated and softened (heating step). The heating temperature is preferredto be equal to or larger than a softening point of a resin forming thesheet 170, but is not limited as long as the heating temperature is atemperature which enables process by transfer described later. Further,the sheet 170 is larger in size than the light guide plate to beobtained in plan view. Further, the thickness of the sheet 170 issubstantially equal to the thickness of the light guide plate to beobtained.

In a second step, a surface structure is transferred onto the sheet 170by depressing a mold to the sheet 170 (transfer step). The mold ispreferred to be a metallic mold made of a metal. The mold has a shapecomplementary to the groove 132 indicated by a line in FIG. 8 on thesurface of the sheet 170, a portion 171 to be formed as the lightintroducing portion, and a portion 172 to be formed as the lightnon-introducing portion, so as to form those members. Therefore, theportion of the mold corresponding to the portion 172 to be formed as thelight non-introducing portion has a protruded shape, and deforms thesheet 170 by crushing the sheet 170. Further, the portion of the moldcorresponding to the portion 171 to be formed as the light introducingportion has a recessed shape, and deforms the sheet 170 so that amaterial at the portion where the sheet 170 is crushed flows therein toform a protrusion. Further, a portion 174 to be formed as the secondportion of the portion 171 to be formed as the light introducing portionhas the same shape as the second portion of the light guide plate to beobtained, but a portion 173 to be formed as the first portion thereofhas a shape longer in the optical axis direction than the first portionof the light guide plate to be obtained. As described above, both theside surfaces of the portion 173 to be formed as the first portion areslightly inclined in a direction in which the width of the portion 173decreases toward the front surface direction of the light guide plate.Note that, in this embodiment, description is made of a case where thegroove 132 is simultaneously formed with the portion 171 to be formed asthe light introducing portion and the portion 172 to be formed as thelight non-introducing portion. In addition, the groove 126 (see FIG. 4)may also be simultaneously formed, or the groove 132 and the groove 126may be formed in different steps from steps of forming the portion 171to be formed as the light introducing portion and the portion 172 to beformed as the light non-introducing portion.

In a third step, the sheet 170 is removed from the mold (removing step).At this time, the sheet 170 may be cooled if necessary.

In a fourth step, the periphery of the sheet 170 is cut out, to therebyobtain the light guide plate (cutout step). The method of the cutout ispreferred to employ a punching process using a cutout blade. The dottedline illustrated in FIG. 8 indicates a cutout line 175 for the punchingprocess. The lens 123 is formed in this step. Further, in thisembodiment, the punching process is employed as the cutout method, andhence the corners of the periphery of the light guide plate to beobtained are slightly rounded for the purpose of facilitating theprocess.

In this step, it is important to correctly align the position of thesurface structure of the sheet 170 formed in the second step describedabove and the position relative to the cutout line 175 for cutting outthe periphery. For this purpose, at an appropriate place on theperiphery of the sheet 170, an alignment mark 176 for positioning isprovided. However, the sheet 170 in this embodiment is very thin andeasily deformed, and further, the accuracy of the cutout process such asthe punching process is not always sufficiently high. Therefore, in thepositioning using only the alignment mark 176, misalignment easilyoccurs particularly between the position of the surface structure of thesheet 170 in the portion 171 to be formed as the light introducingportion and the position relative to the cutout line 175.

Therefore, in this embodiment, the portion 173 to be formed as the firstportion is used to perform mechanical positioning. This positioning isdescribed with reference to FIGS. 9A and 9B. FIG. 9A is across-sectional view taken along the line IXA-IXA of FIG. 8, whichillustrates a state during the cutout process. As illustrated in FIG.9A, the sheet 170 is held so as to keep the sheet 170 still by a holdingmember 177, and under this state, the cutout is performed by a cutoutblade. Portions of the holding member 177 corresponding to the portion173 to be formed as the first portion and the portion 172 to be formedas the light non-introducing portion are formed in shapes complementaryto those members. In particular, in portions of the holding member 177corresponding to both the side surfaces 178 of the portion 173 to beformed as the first portion, inclined surfaces similar to the sidesurfaces 178 are formed. Therefore, when the sheet 170 is held by theholding member 177 from the front side, the positioning of the sheet 170is automatically performed by causing the side surfaces 178 to be incontact with the complementary inclined surfaces. In other words, thesheet 170 has a self aligning function owing to the function of the sidesurfaces 178. With this, correct alignment may be performed between theposition of the surface structure of the sheet 170 particularly in theportion 171 to be formed as the light introducing portion and theposition relative to the cutout line 175. FIG. 9B is a cross-sectionalview taken along the line IXB-IXB of FIG. 8, which illustrates the stateduring the cutout process. The sheet 170 is positioned by being held bythe holding member 177, and is punched out under this state by a cutoutblade 180 fixed to a cutoff tool 179. The planar positional relationshipbetween the cutoff tool 179 and the holding member 177 are correctlymaintained. In this embodiment, an inner surface 181 of the holdingmember 177 serves as a guide of the cutout blade 180, and hence theholding member 177 has an opening in a shape substantially equal to thecutout line 175 in FIG. 8. However, the guide by the inner surface 181is not always necessary, and the opening of the holding member 177 maybe larger than the cutout line 175.

Further, in FIG. 8, the cutout line 175 is set at a position whichpasses through the portion 173 to be formed as the first portion. Thisis because, if the cutout line 175 is set to pass through the portion174 to be formed as the second portion, because the portion 174 to beformed as the second portion has a shape which widens in the directionaway from the light source, when the position of the cutout line 175 ismisaligned or inclined in the optical axis direction (up-and-downdirection in FIG. 8), the shape of the light incident surface 125 (seeFIG. 5) greatly changes. As in this embodiment, when the cutout line 175is at a position which passes through the portion 173 to be formed asthe first portion, because the portion 173 to be formed as the firstportion extends in the direction away from the light source whilekeeping a substantially constant width, even if the position of thecutout line 175 is misaligned or inclined in the optical axis direction(up-and-down direction in FIG. 8), the shape of the light incidentsurface 125 (see FIG. 5) does not greatly change.

Note that, in manufacturing the light guide plate according to thismethod, the one light guide plate may be obtained from the one sheet, orthe light guide plates may be cut out in large numbers from the onesheet, which is so-called multi-panel method. Alternatively, the lightguide plate may be manufactured through batch processing from the sheetin a plate shape, or the sheet in a belt-like form may be wound offsequentially from a roll of raw sheet and the light guide plate may bemanufactured therefrom in sequential processing. In such a case, anemboss roll may be employed as the mold.

FIG. 10 illustrates a modified example of the light entering portion124. FIG. 10 illustrates an example where the front surface of the lightnon-introducing portion 135 is formed as an inclined surface thatsmoothly connects to the light exit surface 121.

Next, description is given of an installation structure for installingthe light-emitting diode 150 in the light guide plate 120. FIG. 11 is aview illustrating an installation structure for installing thelight-emitting diode 150 according to this embodiment in the light guideplate 120. The light-emitting diode 150 is attached to the back side ofthe flexible printed circuits 160, and disposed so as to be opposed tothe light introducing portion 134. The flexible printed circuits 160 isfixed to the light guide plate 120 through an intermediate member 162.Note that, FIG. 11 illustrates merely an outline of the flexiblesubstrate 160 by dashed lines for ease in understanding.

As illustrated in FIG. 11, the intermediate member 162 has a shapecomplementary to the front surface of the light non-introducing portion135, and is fixed to the front surface of the light non-introducingportion 135 at the back surface thereof. Further, the intermediatemember 162 is fixed, at the front surface thereof, to the flexibleprinted circuits 160. The thickness of the intermediate member 162 ispreferred to be substantially equal to a difference between the largestthickness of the light introducing portion 134 and the thickness of thelight non-introducing portion 135. With this configuration, there is noneed to provide a fixing structure between the flexible printed circuits160 and the light introducing portion 134, with the result that thethickness of the installation structure is reduced by the amountcorresponding to the fixing structure.

Note that, in this embodiment, a double sided adhesive tape is employedas the intermediate member 162. However, the present invention is notlimited thereto, and the intermediate member 162 may be made of anyother appropriate material. Further, the intermediate member 162 may notbe required to be uniform in thickness. In a case where the frontsurface of the light non-introducing portion 135 is an inclined surfaceas described above, the intermediate member 162 may be changed inthickness along the inclined surface. Still further, the flexibleprinted circuits 160 illustrated in FIG. 11 is in a rectangular formcovering the light entering portion 124 and the light-emitting diode150, but the present invention is not limited thereto. The flexibleprinted circuits 160 may be in any form as long as being capable offixing the light-emitting diode 150 through the intermediate member 162.

Further, description is given of a structure of a cross section of theliquid crystal display device 100 according to this embodiment. FIG. 12is a cross-sectional view of the liquid crystal display device 100. FIG.12 illustrates positional relations among the optical switching member1, a polarizing plate 4 and a polarizing plate 7 attached to the frontsurface and the back surface, respectively, of the optical switchingmember 1, the light guide plate 120, the prism sheets 112 and 113 andthe diffusion plate 114, which are optical sheets provided on the frontsurface of the light guide plate 120, the reflective sheet 115, thelight-emitting diode 150, and the flexible substrate 160. The opticalswitching member 1 and the flexible substrate 160 are received by thehousing 190 through a light blocking member 164. A spacer 163 may beinserted between the light blocking member 164 and the flexiblesubstrate 160 as necessary, and a space 191 of a predetermined width isprovided at a position corresponding to the display area 9.

The light blocking member 164 has a function of preventing light fromentering the display area 9 from outside, and may preferably employ adouble sided adhesive tape in black. The light blocking member 164 is ina square frame-like form in plan view, and fixes the optical switchingmember 1 and the light guide plate 120 along the entire peripherythereof to the housing 190. In this embodiment, the light blockingmember 164 has an extending portion 165 which is extended like a tonguefrom a position corresponding to the light-emitting diode 150 in theoptical axis direction of the light-emitting diode 150. Then, theextending portion 165 is warped downward in a direction away from theoptical switching member 1 on the back surface side thereof, and fixedto the prism sheet 112 as illustrated in FIG. 12. In other words, theextending portion 165 is indirectly fixed to the light guide plate 120through the prism sheets 112 and 113, and the diffusion plate 114.Alternatively, the extending portion 165 may directly be fixed to thelight guide plate 120. Further, the opposite surface, that is, the frontsurface of the extending portion 165 may be or may not be fixed to thepolarizing plate 7.

When the extending portion 165 as describe above is provided, asillustrated in FIG. 12, the light beam 140 radiated from thelight-emitting diode 150 and leaked out on the front side from theinclined surface 127 is blocked by the extending portion 165 and doesnot reach the display area 9. However, in a case where the light beam140 leaked from the inclined surface 127 on the front side does notcause a problem in quality, the extending portion 165 is not alwaysnecessary, and therefore, the extending portion 165 and the spacer 163may be omitted without any problem.

While there have been described what are at present considered to becertain embodiments of the invention, it will be understood that variousmodifications may be made thereto, and it is intended that the appendedclaims cover all such modifications as fall within the true spirit andscope of the invention.

What is claimed is:
 1. A back light unit comprising: a light guide platemade of a thermoplastic material and including at least one lightintroducing portion and a light exit portion, a front surface of thelight exit portion being a light exit surface, and a back surface facingthe front surface; and a light source facing the light guide plate andemitting light in a light emitting direction, wherein the at least onelight introducing portion includes a first portion and a second portionwhich are connected to the light exit portion, and cylindrical lenseswhich are provided to face the light source; wherein a top surface ofthe second portion has a fan-shaped form in plan view, wherein a topsurface of the first portion has a constant width along a directionperpendicular to the light emitting direction, wherein side surfaces ofthe first portion are inclined with respect to a direction toward thefront surface from the back surface, and wherein a height of the firstportion is the same as a height of the cylindrical lenses.
 2. The backlight unit according to the claim 1, wherein a lens is provided in alight incident surface of the light introducing portion, the lens beingconfigured to refract and scatter incident light.
 3. The back light unitaccording to the claim 2, wherein the at least one light enteringportion includes a plurality of light introducing portion and lightnon-introducing portions at the same side of the light guide plate, thelight non-introducing portions being thinner than the light introducingportions.
 4. The back light unit according to the claim 1, wherein afront width of the first portion is smaller than a back width thereof.5. The back light unit according to the claim 1, wherein the firstportion has a thickness which is thicker than a thickness of the lightexit portion, and the thickness of the second portion is distributed inbetween the thickness of the first portion and the thickness of thelight exit portion.
 6. The back light unit according to the claim 2,wherein the front surface of the second portion is an inclined surface,and an inclination of the inclined surface is greater than aninclination of the front surface of the first portion.
 7. A liquidcrystal display comprising: an optical switching member which includes afirst substrate and a second substrate sandwiching a liquid crystallayer; and a back light unit according to the claim 1 which guides lightto the optical switching member.